(1) Field of the Invention
The present invention relates to a method and apparatus for feedback control of the air-fuel ratio in an internal combustion engine.
(2) Description of the Related Art
Generally, in a feedback control of the air-fuel ratio, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and the detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output signal of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio. According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio required for three way reducing and oxidizing catalysts which can remove three pollutants CO, HC, and NO.sub.x simultaneously from the exhaust gas.
The above-mentioned fuel correction coefficient FAF is affected by the characteristics of the parts of the engine, the environmental changes, and the like. That is, the center of the fuel correction coefficient FAF often deviates from an optimum value such as 1.0 as a result of the individual differences in the characteristics of the parts of the engine such as the air-fuel ratio sensor, the fuel injection valves, the airflow meter (or the pressure sensor), etc., or individual changes due to the aging thereof. Further, the center of the air-fuel correction coefficient FAF deviates from an optimum value when driving at a high altitude. As a result, the difference between the air-fuel ratio correction coefficient FAF during an air-fuel ratio feedback control (closed-loop control) and the air-fuel ratio correction coefficient FAF during a non air-fuel ratio feedback control (open loop control) is large, so that the change of the controlled air-fuel ratio in a transient state between the closed loop control and the open loop control or vice versa is large. Note that the air-fuel ratio correction coefficient FAF during an open loop control is made to be an optimum value such as 1.0.
In order to compensate for the change of the center of the air-fuel correction coefficient FAF due to the individual differences in the characteristics of the parts of the engine, individual changes thereof due to aging, and driving at a high altitude, another air-fuel ratio correction coefficient, called a learning correction coefficient FG, is introduced to maintain an optimum air-fuel ratio. In this case, the base fuel amount TAUP is corrected by two coefficients, i.e., FAF and FG, to obtain a final fuel amount TAU by EQU TAU=TAUP.multidot.FAF.multidot.FG.multidot..alpha.+.beta.
where .alpha. and .beta. are determined by other engine parameters.
In a learning control for calculating the learning correction coefficient FG, it is necessary to consider that vaporized fuel stocked in a canister may be supplied to a combustion chamber under a predetermined condition, thereby temporarily making the air-fuel ratio to be on the rich side. For example, as shown in FIG. 1, which shows the base air-fuel ratio characteristics due to the vaporized fuel from the canister, when the intake air amount Q is within a special range around 100 m.sup.3 /h, the base air-fuel ratio deviates by 10% from the stoichiometric air-fuel ratio (.lambda.=1). Therefore, if the engine is stopped immediately after learning control is carried out for the change of the air-fuel ratio correction amount FAF due to the vaporized fuel from the canister, the air-fuel ratio becomes on the leaner side when the engine restarts. As a result, misfires may be invited to reduce the drivability.
Thus, it is not preferable to perform learning control upon the rich air-fuel ratio due to the vaporized fuel from the canister.
On the other hand, when driving at a high altitude, the density of air becomes small, so that air-fuel feedback control decreases the air-fuel ratio correction coefficient FAF. Therefore, in order to increase the air-fuel ratio correction coefficient FAF, learning control is carried out to decrease the learning correction coefficient FG. As shown in FIG. 2, which shows the base air-fuel ratio characteristics due to the driving at a high altitude, the base air-fuel ratio is at a definite rich value regardless of the intake air amount Q.
In view of the foregoing, it is necessary to discriminate the rich base air-fuel ratio due to the vaporized fuel of the canister from the rich base air-fuel ratio due to the driving at a high altitude.
In FIGS. 1 and 2, note that LL="1" means an idling state of the engine, which will be later explained.